SEDIMENTARY AND METAMORPHIC ROCKS 99 
pages 322, 366. There are many changes going on 
in the strata of various parts of the earth’s crust. The 
most widespread of these is that of decay, to which 
nearly all rocks near the surface are subjected. By 
this action the rocks are usually rendered weaker. 
This change is not the metamorphism which is meant 
here. | 
Although some rocks, such as the igneous, are solid 
at the beginning, many, such as the fragmental and 
organic, are unconsolidated. | Metamorphism begins 
its work by solidifying these, and in some _ places 
it proceeds until they are entirely changed. If, for 
instance, they are subjected to heat, or to the action of 
heated waters, as by the intrusion of a lava mass, they 
may be baked or otherwise changed. This and other 
causes are at work producing metamorphism in the 
earth. 
Results of Metamorphism. — Among the results of 
these changes is the alteration of strata, so that at a 
mere glance their original condition cannot be told. 
Sandstone becomes changed to a dense quartz rock, 
called quartzite, in which the sand grains may no 
longer be visible to the eye; a peat bog may be 
altered to anthracite coal, or even to the graphite, which 
we use in our pencils and which cannot be burned; 
a dense, apparently structureless limestone may become 
transformed to a beautiful white or variegated marble, 
